1. Field of the Invention
The present invention relates to a receiving unit suitable for a CDMA (Code Division Multiple Access) type cellular telephone system, a receiving method thereof, and a terminal unit for use with a radio system thereof.
2. Description of the Related Art
In recent years, a CDMA type cellular telephone system has become attractive. In the CDMA type cellular telephone system, a pseudo-random code is used as a spread code. A carrier of a transmission signal is spectrum-spread. The pattern and phase of each spread code in the code sequence are varied so as to perform a multiple access.
In the CDMA system, the spectrum spread method is used. In the spectrum spread system, when data is transmitted, the carrier is primarily modulated with the transmission data. In addition, the carrier that has been primarily modulated is multiplied by a PN (Pseudorandom Noise) code. Thus, the carrier is modulated with the PN code. As an example of the primarily modulating method a, balanced QPSK modulating method is used. Since the PN code is a random code, when the carrier is modulated by the PN code, the frequency spectrum is widened.
When data is received, the received data is multiplied by the same PN code that has been modulated on the transmission side. When the same PN code is multiplied and the phase is matched, the received data is de-spread and thereby primarily modulated data is obtained. When the primarily modulated data is demodulated, the original data is obtained.
In the spectrum spread method, to de-spread the received signal, the same PN code that has been modulated on the transmission side is required for both the pattern and the phase. Thus, when the pattern and the phase of the PN code are varied, the multiple access can be performed. The method for varying the pattern and the phase of each spread code in the code sequence and thereby performing the multiple access is referred to as CDMA method.
As cellular telephone systems, an FDMA (Frequency Division Multiple Access) system and a TDMA (Time Division Multiple Access) system have been used. However, the FDMA system and the TDMA system cannot deal with a drastic increase of the number of users.
In other words, in the FDMA system, the multiple access is performed on different frequency channels. In an analog cellular telephone system, the FDMA system is usually used.
However, in the FDMA system, since the frequency use efficiency is bad, a drastic increase of the number of users tends to cause channels to run short. When the intervals of channels are narrowed for the increase of the number of channels, the adjacent channels adversely interfere with each other and thereby the sound quality deteriorates.
In the TDMA system, the transmission data is compressed on the time base. Thus, the use time is divided and thereby the same frequency is shared. The TDMA system has been widely used as a digital cellular telephone system. In the TDMA system, the frequency use efficiency is improved in comparison with the simple FDAM system. However, in the TDMA system, the number of channels is restricted. Thus, it seems that as the number of users drastically increases, the number of channels runs short.
On the other hand, the CDMA system has excellent interference resistance. Thus, in the CDMA system, adjacent channels do not interfere with each other. Consequently, the frequency use efficiency improves and more channels can be obtained.
In the FDAM system and the TDMA system, signals tend to be affected by fading due to multi-paths.
In other words, as shown in FIG. 5, a signal is sent from a base station 201 to a portable terminal unit 202 through a plurality of paths. In addition to a path P1 in which a radio wave of the base station 201 is directly sent to the portable terminal unit 202, there are a path P2, a path P3, and so forth. In the path P2, the radio wave of the base station 201 is reflected by a building 203A and sent to the portable terminal unit 202. In the path P3, the radio wave of the base station 201 is reflected by a building 203B and sent to the portable terminal unit 202.
The radio waves that are reflected by the buildings 202A and 203B and sent to the portable terminal unit 202 through the paths P2 and P3 are delayed from the radio wave that is directly sent from the base station 201 to the portable terminal unit 202 through the path P1. Thus, as shown in FIG. 6, signals S1, S2, and S3 reach the portable terminal unit 202 through the paths P1, P2, and P3 at different times, respectively. When the signals S1, S2, and S3 through the paths P1, P2, and P3 interfere with each other, a fading takes place. In the FDMA system and the TDMA system, the multi-paths cause the signal to be affected by the fading.
On the other hand, in the CDMA system, with diversity RAKE method, the fading due to the multi-paths can be alleviated and the S/N ratio can be improved.
In the diversity RAKE system, as shown in FIG. 7, receivers 221A, 221B, and 221C that receive signals S1, S2, and S3 through the paths P1, P2, and P3 are disposed, respectively. A timing detector 222 detects codes received through the individual paths. The codes are set to the receivers 221A, 221B, 221C corresponding to the paths P1, P2, and P3, respectively. The receivers 221A, 221B, and 221C demodulate the signals received through the paths P1, P2, and P3. The received output signals of the receivers 221A, 221B, and 221C are combined by a combining circuit 223.
In the spectrum spread system, signals received through different paths are prevented from interfering with each other. The signals received through the paths P1, P2, and P3 are separately demodulated. When the demodulated output signals received through the respective paths are combined, the signal intensity becomes large and the S/N ratio improves. In addition, the influence of the fading due to the multi-paths can be alleviated.
In the above-described example, for simplicity, with the three receivers 221A, 221B, and 221C and the timing detector 222, the structure of the diversity RAKE system was shown. However, in reality, in a cellular telephone terminal unit of diversity RAKE type, as shown in FIG. 8, fingers 251A, 251B, and 251C, a searcher 252, and a data combiner 253 are disposed. The fingers 251A, 251B, and 251C obtain demodulated output signals for the respective paths. The searcher 252 detects signals through multi-paths. The combiner 253 combines the demodulated data for the respective paths.
In FIG. 8, a received signal as a spectrum spread signal that has been converted into an intermediate frequency is supplied to an input terminal 250. This signal is supplied to a sub-synchronous detecting circuit 255. The semi-synchronous detecting circuit 255 is composed of a multiplying circuit. The semi-synchronous detecting circuit 255 multiplies a signal received from the input terminal 250 by an output signal of a PLL synthesizer 256. An output signal of the PLL synthesizer 256 is controlled with an output signal of a frequency combiner 257. The sub-synchronous detecting circuit 255 performs a quadrature detection for the received signal.
An output signal of the sub-synchronous detecting circuit 255 is supplied to an A/D converter 258. The A/D converter 258 converts the input signal into a digital signal. At this point, the sampling frequency of a controller 254 is much higher than the frequency of the PN code that is spectrum-spread. In other words, the input signal of the A/D converter 258 is over-sampled.
An output signal of the controller 254 is supplied to the fingers 251A, 251B, and 251C. In addition, the output signal of the controller 254 is supplied to the searcher 252. The fingers 251A, 251B, and 251C de-spread the signals received through the respective paths, synchronize the signals, acquire the synchronization of the received signals, demodulate the data of these signals, and detect frequency errors of the signals.
The searcher 252 acquires the codes of the received signals and designates the codes of the paths to the fingers 251A, 251B, and 251C. In other words, the searcher 252 has a de-spreading circuit that multiplies a received signal by a PN code and de-spreads the signal. In addition, the searcher 252 shifts the phase of the PN code and obtains the correlation with the received code under the control of the controller 254. With the correlation between a designated code and a received code, a code for each path is determined.
An output signal of the searcher 252 is supplied to the controller 254. The controller 254 designates the phases of the PN codes for the fingers 251A, 251B, and 251C corresponding to the output signal of the searcher 252. The fingers 251A, 251B, and 251C de-spread the received signals and demodulate the received signals received through the respective phases corresponding to the designated phases of the PN codes.
The demodulated data is supplied from the fingers 251A, 251B, and 251C to the data combiner 253. The data combiner 253 combines the received signals received through the respective paths. The combined signal is obtained from an output terminal 259.
The fingers 251A, 251B, and 251C detect frequency errors. The frequency errors are supplied to the frequency combiner 257. With an output signal of the frequency combiner 257, the oscillation frequency of the PLL synthesizer 256 is controlled.
In the RAKE system, the output signals received through a plurality of paths are demodulated by the fingers 251A, 251B, and 251C. The output signals of the fingers 251A, 251B, and 251C through the respective paths are combined by the combiner 253. In the related art reference, when the output signals through the respective paths are combined, after the demodulated output signals through the respective paths are matched on the time axis, they are simply combined.
As described above, in the related art reference, output signals through respective paths are simply combined. When the output signals through all the paths are combined, the S/N ratio is improved and thereby the received signal is prevented from being affected by fading.
However, correctly demodulated output signals are not always obtained through respective paths. In particular, a demodulated output signal of a weak signal received through a path may have many errors. In this case, when there is a path in which many errors take place, if the demodulated output signal received through the path is combined, errors increase in the combined signal.